Orthogonal frequency-division multiplexing (OFDM) is a special multi-carrier modulation (MCM) technique in which a single data stream is transmitted over a number of lower rate orthogonal sub-carriers. An OFDM signal is generated from a serial stream of binary digits by dividing the input stream into N parallel streams. Each stream is mapped to a symbol stream using a modulation scheme, such as QAM (Quadrature Amplitude Modulation) or PSK (Phase-Shift Keying). An inverse fast Fourier transform (IFFT) is computed on each set of symbols to transform a set of sub-carriers into a time-domain signal. Each set of symbols transmitted at the same time is referred to as an OFDM symbol.
FIG. 1 illustrates an example OFDM frame 102 which comprises a block of OFDM symbols 104. To eliminate or reduce inter-symbol interference (ISI) a guard interval (GI) 106 is appended at the start of the data portion 108 of each symbol 104. As long as echoes fall within this interval, they will not affect the receiver's ability to correctly decode the symbol. Generally a cyclic prefix consisting of the end portion 110 of the data portion 108 of the symbol is transmitted during the guard interval 106.
Each OFDM symbol is decoded at an OFDM receiver by performing a FFT (Fast Fourier Transform) on the data portion 108 of the symbol 104. However, since the guard interval 106 includes a duplicate of the end portion 110 of the data portion 108 the FFT can be performed on any portion of the symbol equal to the data length. This is because any portion of the symbol equal to the data length will comprise all of the data. The portion of the symbol on which the FFT is performed is referred to as the FFT window. This may also alternatively be referred to as the FFT symbol window.
Accordingly, as shown in FIG. 2, the start of the FFT window for a particular symbol 104 may be positioned anywhere between the start 202 and end 204 of the guard interval 106. In particular, the FFT window may be at position A 206, position B 208 or anywhere in between (e.g. position C 210). The start 202 of the guard interval 106 may also be referred to as the starting edge, leading edge or the first edge of the guard interval 106. Similarly the end 204 of the guard interval 106 may also be referred to as the ending edge, following edge or the second edge of the guard interval 106.
In wireless OFDM systems an OFDM signal generated by a transmitter will typically reach the receiver via many different paths. For example, as shown in FIG. 3, the OFDM receiver may receive a primary or main signal 302 comprising symbol A; and a secondary signal or echo 304 also comprising symbol A, which is positively or negatively delayed in time with respect to the strongest main signal 302. It is advantageous to position the FFT window so the most amount of energy can be obtained from both the main signal 302 and the echo 304. For example, in FIG. 3, ideally the FFT window is positioned in the period 306 during which the main signal 302 and the echo 304 overlap (i.e. from the start of the main signal 302 until the end of the echo signal 304) so that symbol A can be correctly decoded via the main signal 302 and the echo 304. Accordingly, in the example of FIG. 3, the FFT window is preferably positioned at position A 308, position B 310 or anywhere in between (e.g. position C 312).
It is important to position the window correctly because distortions can arise by mis-positioning the FFT window. However, determining the correct location for the FFT window is difficult since there are typically many echoes or secondary signals, some of which may be quite weak (e.g. below 20 dB).
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known OFDM receivers.